The present invention relates to a color cathode ray tube (CRT) device of the in-line type, in which the positional arrangement of a dynamic convergence correcting magnetic field generating component (referred to as a dynamic convergence component, hereinafter) relative to the electron gun is improved.
In conventional color CRT devices of the in-line type, the distribution of the magnetic field generated by a deflection yoke is made suitably non-uniform, and a magnetic field control element incorporated in an end portion of the electron gun together with the suitable non-uniform magnetic field distribution converge these in-line electron beams onto an image surface of the tube. However, possible variations of the magnetic field generated by the deflection yoke, the magnetic field distribution of the in-line arrangement of the three electron beams emitted from the electron gun, of the electron beam and the deflection yoke in combination as well as possible assembly errors in mass-production have caused precise convergence of the three electron beams throughout the surface of the image plane to be impossible. In general, the amount of convergence error to be further corrected in a mass-produced color CRT device of this type is about 0.5-1.0 mm.
Particularly, when a color CRT device of the in-line type is used as a display device of a computer terminal, color deviations around peripheral portions of the image plane cause characters displayed on the CRT image plane in different colors to deviate from each other and thus the quality of the CRT device as a display medium is degraded.
In order to correct this convergence error and hence eliminate the color deviation problem around the periphery of the tube surface, it has been proposed to arrange the dynamic convergence component at the outer periphery of a neck portion of the color CRT. This proposal has been realized in various manners.
FIG. 1 is a schematic illustration of the neck portion of a color CRT device 1 of the in-line type, which is constructed according to a typical example of the above proposal. In FIG. 1, a three beam electron gun 3 for producing three in-line electron beams 31, 32 and 33 is incorporated in the neck portion 2. Various voltages are applied through a base portion 4 to the electron gun 3 to cause the latter to emit the beams 31, 32 and 33. These electron beams are passed through a deflection yoke 5 which produces a specific non-uniform magnetic field distribution and are deflected horizontally and vertically towards given points on the image surface.
A static convergence correction magnetic field generating component (referred to as a static convergence component hereinafter) composed of two, four and six pole magnets is provided around the outer periphery of the electron gun 3, in which the three electron beams 31, 32 and 33 are corrected in convergence error around a central portion of the image plane and converged to a point in the central area by regulating the magnetic field strengths of two four-pole magnets and two six-pole magnets. Color purity correction at the image plane is also performed by regulating the magnetic field strength of the two-pole magnet. The three beams 31, 32 and 33 to be converged to a point in the central area of the image plane pass through the magnetic field produced by the deflection yoke 5 and are deflected horizontally and vertically. It is required that the three beams be convergable at any point whether at peripheral or central areas of the image plane.
In order to realize the above, the magnetic field produced by the deflection yoke 5 should be distributed non-uniformly in a specific horizontal and vertical pattern and a coma correction should be provided. The coma correction is performed by a coma correcting magnetic field control component (referred to as coma control component hereinafter) 7 provided at an end of the electron gun 3 so that a deflection sensitivity correction of the center beam 32 and the side beams 31 and 33 may be performed.
In order to further improve the convergence preciseness of the beams 31, 32 and 33 in the peripheral areas of the image plane, the side beams 31 and 33 are corrected by the dynamic convergence component 8 provided between the deflection yoke 5 and the static convergence component 6 so that these beams 31 and 33 are overlapped on the central beam 32. Therefore, the amount of convergence to be corrected can be reduced to an amount smaller than 0.5 mm.
The dynamic convergence component 8 is composed of two four-pole magnetic field generating elements 81 and two six-pole magnetic field generating elements 32 as shown in FIGS. 2A and 2B, respectively. The elements 81 are composed of a ferrite core ring 83 and two sets of four coils 85 wound equiangularly on the ring 83, the sets of coils 85 being off-set in phase by 45.degree. from each other as shown in FIG. 2A, and the elements 32 are composed of a similar ferrite core ring 84 and two sets of six coils 86 wound equiangularly on the ring 84, the sets of the coils 86 being off-set in phase by 30.degree. from each other. It should be noted that the core ring 84 may be eliminated and instead, the core ring 83 may be used concurrently. The convergence correction is performed by varying the magnetic field strengths produced thereby by regulating the currents flowing through the coils 85 and coils 86, respectively.
However, the correction sensitivity of the element 82 of the dynamic convergence component 8 is very low and therefore a large amount of current must be supplied to the coils 85 causing the correction cost to be very high. Furthermore, the heat generated in the coils 86 due to the large amount of current flowing therethrough causes a drift in the dynamic convergence correction.